Abscesses are circumscribed collections of purulent material (pus) found in several species of animals in a variety of locations. This purulent inflammation is usually caused by one of four pyogenic (pus producing) bacteria: Trueperella, Pseudomonas, Streptococcus and Staphylococcus. Abscesses formed in mammals generally contain white, green or yellow creamy material whereas, because of the high body temperature of birds, their abscesses generally have a caseous (cheesy) exudate, are walled-off and are painless. An abscess may be acute or chronic, focal or multiple, and may range in size from microscopic to unlimited dimensions. An abscess occurring on the footpads of avian species is called bumblefoot.
Abscesses are found in mammals, birds and reptiles throughout North America. In Michigan, abscesses are usually seen in white-tailed deer, but have also occurred in beaver, Eastern fox squirrel, snowshoe hare, raccoon, muskrat, red-tailed hawk, great horned owl, cottontail rabbit, black squirrel, mink, ruffed grouse, mute swan, trumpeter swan, elk, moose, Canada geese, wood duck, wild turkey, common loon, mallard, Cooper's hawk, snowy owl, double-crested cormorant, ring-necked pheasant, great blue heron, Sichuan pheasant, mourning dove, American robin, striped skunk, woodchuck and opossum.
Bumblefoot has been observed in great horned owl, sandhill crane, bald eagle, red-tailed hawk and Hungarian partridge.
Transmission and Development
In mammals, injuries such as gunshot or arrow wounds, or wounds acquired while fighting, will often result in abscess formation.
In birds, abscesses may arise from infected wounds, injuries to the foot pads (bumblefoot), damaged feather follicles, blocked sebaceous glands, pressure and friction points, or in the area beneath damaged skin. Abscesses may also be seen in the liver and spleen of avian species.
The above mentioned injuries may be the result of either physical or chemical damage. These injuries produce tissue damage which causes cells to die in the center of the injured area. This area is then invaded by pyogenic bacteria which produce a purulent (suppurative or pus-forming) inflammation. The cells become liquefied by the proteolytic enzymes which are present in the area. The liquefied area is encircled by a cavity, walled off by a combination of fibrous connective tissue, proliferating capillaries and leukocytes.
The purulent exudate may be green, blue-green, yellow, white, red or black in color. Certain bacterial species typically cause characteristically colored pus: Trueperella (green), Pseudomonas (blue-green), Streptococcus and Staphylococcus (yellow or white). Pus from an abscess found in a canid is usually of a thin and watery consistency while an abscess from a ruminant has viscid pus. Avian pus is usually dry and caseous due to the presence of an antitryptic enzyme.
If an abscess is not firmly enclosed and consequently remains active, it will protrude into surrounding tissues along the route of least resistance. This is due to the increased pressure caused by the addition of material to the abscess. If the abscess enters a muscle sheath or along muscle planes, as often occurs in deer, it will spread along the muscle layers for a considerable distance. If the abscess reaches the surface of the skin, an ulcer or open sore will form and will emit purulent material onto the skin.
Clinical Signs and Pathology
In mammals the effects of an abscess are generally non-existent, as the material is usually localized. Abscesses that spread along the muscle layers, or metastasize into various organs may cause pathological conditions that are hazardous to the animal's well being.
In birds, abscesses usually form enlarged spherical areas that may hinder movement or feeding ability. Often the feet are involved (bumblefoot) and the enlarged areas result in an inability to stand and to capture prey.
The appearance of considerable numbers of neutrophilic leukocytes in or on a tissue justifies a diagnosis of purulent inflammation. Culturing the abscess material for members of the genera Trueperella, Pseudomonas, Streptococcus and Staphylococcus confirms the diagnosis and the causative organism.
Treatment and Control
In mammals the usual treatment would consist of drainage and antibiotic therapy. In time, abscesses may become inactive or enclosed (sterile); the body defenses having killed all of the causative bacteria. The accumulated pus, with no route of escape, will slowly become liquefied and be absorbed.
In birds the abscess must be opened and the accumulated pus scraped out manually. The abscessed area should then be cleaned with an antiseptic solution and wound medication placed on the site. Bandaging of the wound will aid in preventing further infection.
An abscess is a localized infection which may result in pain, tissue damage, septicemia and death. However, in most cases, an animal's defense mechanisms take care of abscesses with little or no long-term effects.
Concerning the edibility of the affected animal, generally, if the abscessed area is removed, it is safe to consume the remaining meat. If, however, the abscessed area is widespread, has an offensive odor, or is aesthetically displeasing, it may be better to refrain from consuming the affected meat.
Michigan DNR Wildlife Disease Laboratory.